Adapting reference signal density

ABSTRACT

A wireless communications network is configured to transmit signals including a plurality of data units containing reference signals for use in channel assessment by mobile stations receiving the transmissions. Topological data relating to the relative positions of each base stations in the network is used by a density adjustment function to select a distribution pattern and density of reference signals. A signal is sent to one or more user terminals, the signal comprising reference signals arranged in the selected distribution and density of plurality of data units, and including a control signal to indicate to the mobile terminal which density and distribution is in use.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a National Phase entry of PCT Application No. PCT/EP2017/052867, filed Feb. 9, 2017, which claims priority from EP Patent Application No. 16157949.5, filed Feb. 29, 2016, each of which is hereby fully incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to the operation of cellular radio systems with multiple small cells, such as femtocells, typically installed inside buildings where the distances between the base station (eNodeB) and the user terminals are not expected to be greater than about 100 m and the maximum speed of the user terminals is not expected to be greater than about 30 km/hr.

BACKGROUND

In the “long term evolution” (LTE) standard, reference signals (also known as pilot signals) are constantly broadcast by the base stations. The user terminals receiving them use them to assess the properties of the downlink channel. The LTE system uses different kinds of reference signals, sent from different antenna ports and performing different tasks. These include cell-specific reference signals (or common reference signals), user terminal-specific reference signals (or demodulation reference signals), positioning reference symbols and channel state reference signals.

User terminals estimate the downlink channel characteristics in order to perform equalization before demodulation of the data symbols. If the UE is moving, there will be time and frequency domain effects as follows:

-   -   The time domain effect is caused by multipath reflections,         whereby the reflections cause disturbances in the amplitude and         phase of the signals. Such reflections come from surfaces near         to the UE and more distant from the UE, and the difference in         time of arrival of the reflected signals generally increases         with high eNodeB transmit powers and longer distances between         the eNodeB and UE.     -   The frequency domain effect is caused by Doppler shift, whereby         the frequency of the received signal at the eNodeB will be         shifted down if the UE is moving away from it and shifted up if         the UE is moving towards it.

This combination of multipath reflections and the Doppler effect produce an effect known as Doppler spreading. So that the UE can correct for these impairments, the base station repeats the reference signals at regular intervals of time (typically 0.5 ms) and of frequency (typically 100 kHz), spreading across the transmitted bandwidth, to allow a UE to detect sufficient reference signals to reconstruct the channel. The reference signals can be thought of as sampling the channel in both the time and frequency domain. The user terminal uses the standard properties of the reference signals to calculate the amplitude and phase correction that should be applied to every non-reference symbol, and it interpolates between the reference signals as part of the calculation.

The standard LTE reference signal pattern is suited to macro-cellular systems where the UE can receive reflections from many kilometers away (a 3 km round trip is a delay of approximately 1 millisecond), and still rebuild the time domain aspects of the channel, and where the UE can be travelling at 300 km/hr and still rebuild the frequency domain aspects of the channel. The reference signals are transmitted all of the time and comprise about one seventh of the maximum output power of the base station.

For small cells, such as indoor femtocells, both the range and speed of user terminals are much smaller. It is therefore possible to reconstruct the channel in both the time and frequency domain with a very much smaller number of samples or reference signals, which reduce the power requirements of the base station, and increases scope to mitigate reference signal pollution, because each base station is using less spectrum for its reference signals.

Significant power savings and efficiency improvements can be realized by adapting the reference symbol density dynamically in this way. Examples are described in, for example, U.S. Patent Application Publication No. 2016/0020877. However, for this to be done efficiently, the base station needs to be able to determine what reference signal density is appropriate. One factor in determining a suitable reference signal density may be the degree of difficulty expected to be experienced by mobile terminals in initially acquiring the channel transmitted by the base station.

SUMMARY

According to a first aspect of the disclosure, there is provided a method of operating a wireless communications network comprising a plurality of user terminals and a plurality of base stations, the locations of the base stations being dynamically variable, wherein the base stations transmit signals comprising a plurality of data units containing reference signals for use in channel assessment by mobile stations receiving the transmissions, the method comprising:

-   -   maintaining data in a network management system relating to the         base stations' relative positions;     -   selecting, by the network management system, a reference signal         pattern density for one or more of the base stations in         accordance with the position data, and selection, by the network         management system, from a predetermined set of distribution         patterns of reference symbols a distribution pattern having the         selected pattern density over the plurality of data units,     -   the network management system causing each base station to         generate a signal configured according to a first protocol for         transmission to one or more user terminals, the signal         comprising reference signals arranged in the distribution and         density of the plurality of data units selected for that base         station, and         -   transmitting instructions from the network management system             to one or more base stations modifying the selected             reference signal densities for the base stations in response             to changes in the maintained data relating to the relative             positions of the base stations.

The disclosure also extends to a computer program element comprising computer program code to, when loaded into a computer system and executed thereon, cause the computer to perform the method specified above, and also extends to apparatus configured to perform such a method.

A second aspect of the disclosure provides a wireless communications system comprising a plurality of base stations configured to transmit signals comprising a plurality of data units, the locations of the base stations being dynamically variable, wherein the base stations have signal transmission means for transmitting signals comprising a plurality of data units containing reference signals for use in channel assessment by mobile stations receiving the transmissions, wherein each base station has means to generate a signal configured according to a first protocol for transmission to one or more user terminals, the signal comprising reference signals arranged in a distribution and density of the plurality of data units selected for that base station, a network management system of the wireless communications system comprising a density selection processing system for selecting a density for the data units in the transmissions of respective base stations, the network management system comprising:

-   -   means for maintaining data relating to the relative positions of         the base stations;     -   density selection means for selecting, for one or more of the         base stations, a distribution pattern from a predetermined set         of distribution patterns of reference signals, having a         respective pattern density selected in accordance with the         position data,     -   instruction generation means for causing the or each base         station to transmit a signal comprising reference signals         arranged in the selected distribution and density of plurality         of data units,     -   and means for transmitting instructions to one or more of the         base stations to modify their selected reference signal         densities in response to changes in the maintained data relating         to the relative positions of the base stations, and transmitting         instructions to the base stations from the network management         system.

The signal may include a master information block of a data frame characterizing the signal. The reference signal distribution in each of the set of distribution patterns may be arranged to be substantially the same in a region of a data frame adjacent to a master information block, and varies in regions of the data frame remote from the master information block.

The density adjustment means may be comprised in a network management system controlling a plurality of the base stations, or it may be comprised in one or more of the base stations, the base stations having means to retrieve network topology data from a database maintained by a network management system. Alternatively, it may be comprised in one or more of the mobile terminals, the mobile terminals having means to retrieve network topology data from a database maintained by a network management system.

The disclosure therefore allows adaptive changes to the density of reference signals according to cell size, determined by the proximity of the cell to others in the system. Small cells are typically associated with indoor locations, or venues where users tend to spend some time, where the users are unlikely to be moving at speeds likely to make Doppler shifts significant. Moreover, because the cells are small, the path lengths are small and any echoes causing variations in path lengths and therefore time synchronization also correspondingly small.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the disclosure will now be described, by way of example and with reference to the drawings, in which:

FIG. 1 is a schematic illustrating the physical elements which co-operate in the performance of this embodiment of the disclosure.

FIG. 2 is an illustration of synchronization signals in a physical broadcast channel containing a master information block (MIB) in an LTE frame according to section 5.2 of TS36.331, release 13 of the 3GPP standard.

FIGS. 3A, 3B and 3C are enlarged views of part of the frame depicted in FIG. 1, illustrating different densities of reference signals.

FIG. 4 is a flow chart depicting the process of configuring the reference signal density.

DETAILED DESCRIPTION

FIG. 1 depicts the functional elements operable in the physical components which co-operate in this embodiment. It will be understood that the functional elements may be embodied in software or firmware in the physical devices, and the various functions may share physical elements with each other or with other functions not depicted herein.

A user terminal 20 is in wireless communication with a base station (eNodeB) 21, which is under the control of a network management system 22. It will be understood that in a typical system several base stations will be managed by a network manager, and several user terminals can be in communication with each base station. Only the functions relevant to the operation of this embodiment are depicted in FIG. 1: there would also be standard functions such as power management, coding and decoding, call logging and, in the user terminal, a user interface.

The user terminal functions include an RF interface 200 through which the terminal communicates with base stations such as the one depicted at 21. The functions of the terminal include a synchronization module 201 which monitors transmissions received over the RF interface 200 from the base station 21, to detect synchronization data, and controls transmissions from the user terminal such that they are synchronized with the operation of the base station. There is also a channel estimation function 202 which monitors reference signals and control signals received over the RF interface 200 from the base station 21 in order to determine properties such as signal quality, Doppler shift, etc. The results of such analysis are used by a report generation function 203 to generate a report for transmission via the RF interface 200 to the base station 21. In addition, a physical cell ID interference checking function 204 is provided, which also provides an input to the report function 203.

The base station 21 also comprises an RF interface 210 which communicates with user terminals such as the one depicted at 20. The functional components which contribute to the operation of this embodiment include a location reporting system 211 which co-operates with the network management system 22 to determine the physical location of the base station, in particular in relation to other base stations. The reporting system may make use of absolute position measurement systems like GPS, also detection of other wireless transmitters such as other base stations, and movement detectors which may, for example, indicate that the base station is in a moving vehicle.

The base station includes a signal quality reporting function 212 which, in co-operation with the channel estimation function 202 in the co-operating mobile stations (e.g. 20) can be used to determine handover decisions, selection of appropriate bandwidth and error checking modes, etc. The signal quality reporting function also provides an output to the network manager 22.

A further function of the base station 21 is a density adjustment control 214. The base station 21 may control this autonomously in response to the signal quality reports 212, or it may be controlled by instructions received from a density adjustment function 221 of the network manager 22. The density adjustment control system controls the signal generation system 213, specifically by giving instructions on the density of reference symbols to be used in transmission to the user terminals 20. The signal generation system 213 is responsive to this input firstly to generate reference symbols at the required density, and secondly to include a control signal in the master information block (MIB) indicating what density is being used.

The network management system 22 comprises a store 220 of the network topology. This may be maintained manually for planned network changes, such as the erection of a new fixed base station, or a change of channel allocations. However, modern networks include a large number of unstructured changes as users connect and disconnect portable base stations (femtocells) which are not under the direct control of the network operator, and as base stations on moving vehicles such as railway trains move relative to the fixed network. The topology store 220 therefore also uses reports received from the location reporting systems 211 of such base stations 21 to update the topology record.

The network management system also comprises a density adjustment function 221 which receives signal quality and interference reports from the various base stations and returns instructions to their density adjustment functions 214 when a change of reference symbol density is required.

FIG. 2 illustrates a typical LTE data frame. Time is represented on the horizontal axis and frequency on the vertical axis. In the frequency domain, the frame has a bandwidth of 3 MHz divided into fifteen resource blocks (RBs), each resource block containing twelve subcarriers. In the time domain, the frame has a duration of 10 ms divided into ten subframes, each containing two time slots (so twenty slots in total), each subframe containing seven symbols. A master information block 110 contains data defining the channel, in particular the physical cell ID (PCI). This block 110 takes up part of seven resource blocks (labeled 101) over part of the second time slot. An inset to FIG. 1 depicts this block in more detail.

Synchronization symbols 111, 112 occur in the first and eleventh time slot, in the same resource blocks as the MIB.

Reference signals are distributed in a regular pattern over the frame, typically two in each time slot in each resource block. The reference signals are shown in FIG. 2 as solid elements (three of them indicated at 120), which occur every seventh symbol in the frequency axis (vertical) and every third and fourth symbol alternately in the time axis (i.e. symbols 1,4,8,11,15 etc.). It will be seen that this pattern continues throughout the frame, including within the block 110 otherwise occupied by the MIB.

FIG. 3A is an enlarged view of the area of FIG. 1 marked 100, including part of the MIB 110 and the synchronization symbols 111. FIGS. 3B and 3C depict the corresponding area of modified blocks in which the density of reference signals has been reduced. In FIG. 3B the reference signals have been reduced by 25% and in FIG. 3C they have been reduced by 75%. In both cases, the reduction in density only takes place in the resource blocks (frequency bands) 102 that are furthest from the centre frequency. The blocks 101 closest to the centre frequency contain the MIB 110, and the reference symbols in that region are required for the UE to estimate the channel in order that it can decode the MIB. The reference symbol density does not need to be uniform across the non-central symbol area 102, as different UEs can be served from different parts. As depicted in FIGS. 3B and 3C, the density is reduced further in subcarriers more remote from the MIB.

It is envisaged that a few patterns of reference signals, say ½, 1/10, 1/20, 1/100 of the maximum density would be available, and a signaling protocol developed to identify the pattern currently in use by the base station, to which the user terminals would operate. In this embodiment, the MIB contains a message informing the terminals whether it is using a sparse or a full set of cell-specific or other reference signals. The MIB as defined in the LTE standard (section 5.2 of 3GPP TS36.331, release 13) has ten spare bits. Four patterns (including the standard density) can be represented by two binary digits in the MIB. A third digit would allow up to eight patterns.

Referring to FIG. 1 and FIG. 4, the process operates as follows:

When a User Terminal 20 synchronizes with a base station 21 (step 50), the synchronization function 201 locks onto the Primary and Secondary Synchronization signals (111,112) transmitted by the base station, in order to deduce the physical cell ID (PCI). After that, the user terminal can make use of the cell-specific reference signals 120 because these reference signals can be descrambled using the PCI. The reason the reference symbols are scrambled for transmission is so that the UE can discriminate between cell-specific reference signals even in areas of overlap between two cells, provided that the PCIs are different.

The user terminal then performs a channel estimation 202 in the region of the Master Information Block (MIB) on the Broadcast Channel (BCH)—at 51. The MIB carries information relating to the bandwidth used by the channel and this is read by the terminal. In this embodiment the MIB includes an indicator of the signal density, and the channel estimation function 202 reads this. From the indicated density, the channel estimation function 202 can determine which symbols in the frame may be used as reference symbols (at 52). It will be understood that, as a symbol location may be used as a reference symbol in some density patterns and not in others, it is necessary for the mobile terminal to identify which pattern is in use in order to determine where the reference symbols are to be found.

Several mechanisms can be used, together or in combination, to set the required density, and these are the subject of the present application and the Applicant's two co-pending International applications, claiming priority from UK Application Nos. 1603500.8 and 1603506.5, respectively, and European Application Nos. 16157948.7 5 and 16157950.3, respectively, both filed contemporaneously with this one.

The reference symbol density can be controlled by a density adjustment function 214 in the base station 21 which is responsive to signal quality measurements 212 to control the signal generation function 213 which generates the data frames to be transmitted over the RF interface 210. In particular the density adjustment function 214 instructs the signal generation system 213 where to place the reference symbols, and to apply the required control signal in the MIB to indicate the density in use.

A density adjustment function 221 in the network management system 22 can provide more coordinated control than is possible if each base station acts autonomously, by acting in response to data 220 relating to the deployment topology and reports transmitted from the base station's signal quality monitoring systems 212 relating to the radio environment (at 56). The topology data 220 can be updated from time to time using a reporting function 211 (at 570), or by manual intervention if the topology changes for example by addition or removal of a cell.

The wireless environment monitoring function 212 monitors signals received over the RF interface 210 and reports changes in quality to the density adjustment function 221 in the network management system 22. The network manager 22 queries the radio environment database 220 periodically and selects a suitable reference signal density (at 57). Changes can be triggered by changes in the topology identified by the cell location database or by the mobility management entity (MME). Such changes would include addition or removal of cells, whether by physical movement of a device or by switching it on or off. As has already been discussed, a higher density is required in conditions where Doppler spreading is to be expected, to ensure that the mobile terminal can lock on to one of them. In particular, where base stations are widely spaced and/or in outdoor locations, there are likely to be more long-path reflections and fast-moving mobile terminals, both of which can contribute to channel-spreading effects, in time and frequency, respectively.

Changes in reference symbol density selected by the density adjustment function 221 in the network management system (at 57) are transmitted as instructions to the base station (at 58), where they are used by the density adjustment function 214 to control the signal generation function 213 which generates the data frames to be transmitted over the RF interface 210. In particular the density adjustment function 214 instructs the signal generation system 213 where to place the reference symbols, and to apply the required control signal in the MIB to indicate the density in use.

Another mechanism (at 53-56) is operated by co-operation between terminals and the network. Changes using this mechanism can be triggered if the channel estimation system 202 in a User Terminal 20 is itself able to detect when it is not able to estimate the channel accurately enough, by comparing the BLER in relation to RSRP and RSRQ.

Terminal performance is monitored, for example by detecting an inability to adequately estimate the channel, which can be measured either by the terminal itself or by the network (at 53). One way that the UE or the network can detect when the UE is not able to estimate the channel accurately enough is a high level of block error rate (BLER) when the received signal is strong enough to give a much lower BLER. The symptoms of a high BLER while the RSRP and RSRQ measurements are healthy may indicate an insufficient density of reference signals.

If the error rate is well within acceptable limits the symbol density may be reduced (at 555).

If the UE is failing to estimate the channel, remedial action can be taken, but the cause needs to be identified to determine the appropriate action to take. In particular, a PCI conflict can occur when overlapping cells have the same PCI (physical cell identity), and this can also be a cause of a high BLER, since it would impair the UE from properly estimating the channel as the reference signals are scrambled by another signal using the same PCI. In this embodiment, having identified a high error rate (at 54) the system checks that identity conflict is not the cause (at 55) before the base station is reconfigured to increase the reference signal density (at 56). If a physical cell identity conflict is identified, a self-organizing network (SON) process is invoked (at 560) to initiate a change to the identity code, and the monitoring process 53 is repeated.

It will be noted that if PCI conflict is the cause, increasing the density of the reference symbols would be likely to increase the number of such conflicts, thereby exacerbating the problem. It is therefore desirable to determine whether a PCI conflict is the cause before increasing the reference symbol density.

In the absence of any PCI impairment reports, the report generator 203 transmits a report through the RF interfaces 200, 210 to the signal quality report generator 212, allowing the base station 21 to report to the network manager 22 that an increase in density should be attempted (at 56). The network manager's density adjustment management system 221 can then select a suitable reference signal density (at 57).

When the network manager determines that a change in reference signal density is required (at 57) it passes an instruction to the base station 21 (at 58) causing its density adjustment management system 214 to reduce the number of reference signals generated by the signal generation system 213 according to a predetermined pattern, and to cause the signal generation system 213 to amend the MIB 110 to indicate the pattern of reference signals now in use (at 58). The channel estimation system 202 in each User terminal 20 connected to the base station 21 periodically reads the MIB and so will identify the revised reference signal pattern (at 59), and read the reference signals from the pattern currently being used (at 52).

The process of changing reference symbol density is of course reversible, as changes in network topology or channel estimation values indicate that an increase in density is necessary to maintain signal quality, or that a decrease in density can be achieved without unacceptable impairment in signal quality. 

1. A method of operating a wireless communications network comprising a plurality of user terminals and a plurality of base stations, locations of the plurality of base stations being dynamically variable, wherein the plurality of base stations transmit signals comprising a plurality of data units containing reference signals for use in channel assessment by mobile stations receiving the transmissions, the method comprising: maintaining position data in a network management system relating to relative positions of the plurality of base stations; selecting, by the network management system, a reference signal pattern density for one or more of the plurality of base stations in accordance with the position data, and selecting, by the network management system, from a predetermined set of distribution patterns of reference signals, a distribution pattern having the selected reference signal pattern density over the plurality of data units, the network management system causing each of the plurality of base stations to generate a signal configured according to a first protocol for transmission to one or more user terminals, the signal comprising reference signals arranged in the distribution pattern and the reference signal pattern density of the plurality of data units selected for that base station, transmitting instructions from the network management system to one or more of the plurality of base stations modifying the selected reference signal pattern densities for the one or more base stations in response to changes in the maintained position data relating to the relative positions of the plurality of base stations.
 2. The method according to claim 1, wherein the signal includes a master information block of a data frame characterizing the signal.
 3. The method according to claim 2, wherein reference signal distributions in each of the set of distribution patterns are substantially similar in a region of a data frame adjacent to the master information block, and varies in regions of the data frame remote from the master information block.
 4. An apparatus configured to perform the method in accordance with claim
 1. 5. A non-transitory computer-readable storage medium storing a computer program element comprising computer program code to, when loaded into a computer system and executed thereon, cause the computer to perform the method as claimed in claim
 1. 6. A wireless communications system comprising: a plurality of base stations configured to transmit signals comprising a plurality of data units, locations of the plurality of base stations being dynamically variable, wherein the plurality of base stations have signal transmission means for transmitting signals comprising a plurality of data units containing reference signals for use in channel assessment by mobile stations receiving the transmissions, wherein each of the plurality of base station has means to generate a signal configured according to a first protocol for transmission to one or more user terminals, the generated signal comprising reference signals arranged in a distribution and a density of the plurality of data units selected for that base station; and a network management system comprising a density selection processing system for selecting a density for the data units in the transmissions of respective base stations, comprising: means for maintaining position data relating to relative positions of the base stations, density selection means for selecting, for one or more of the plurality of base stations, a distribution pattern from a predetermined set of distribution patterns of reference signals, having a respective pattern density selected in accordance with the position data, instruction generation means for causing the or each base station to transmit a signal comprising reference signals arranged in the selected distribution and density of plurality of data units, and means for transmitting instructions to one or more of the plurality of base stations to modify selected reference signal densities in response to changes in the maintained position data relating to the relative positions of the base stations, and transmitting instructions to the base stations from the network management system.
 7. The wireless communications system according to claim 6, wherein the transmitted signal includes a master information block of a data frame characterizing the signal.
 8. The wireless communications system according to claim 6, wherein the reference signal distribution in each of the set of distribution patterns is substantially the same in a region of a data frame adjacent to a master information block, and varies in regions of the data frame remote from the master information block.
 9. The wireless communications system according to claim 6, wherein the density selection means is comprised in the network management system controlling a plurality of the base stations
 10. The wireless communications system according to claim 6, wherein the density selection means is comprised in one or more of the plurality of base stations, the plurality of base stations having means to retrieve network topology data from a database maintained by the network management system.
 11. The wireless communications system according to claim 6, wherein the density selection means is comprised in one or more of the mobile terminals, the mobile terminals having means to retrieve network topology data from a database maintained by the network management system. 